1. Field of the Invention
The present invention, generally, relates to print hammer devices for use in high speed printing apparatus and, more particularly, to a new and improved system for lubricating the pivot pin for such print hammer devices.
Although movement of a print hammer of the type with which the present invention is adapted to function is limited to a degree or so, the prior art of bearings for rotatable motors, generators and the like, reveal structure that is helpful in developing an understanding of the full scope of the invention. In addition, this prior art is useful to illustrate the problems solved by the invention.
2. Description of the Prior Art
There are bearing assemblies taught in the prior art that have a lubricant reservoir for holding a reserve supply of a lubricant in various forms, such as felt pads or washers. The use of felt pads or washers positioned to engage a bearing to release the lubricant by capillary action requires a mechanically rigid enclosure to hold the felt in position.
Prior U.S. Pat. No. 3,138,412 teaches the use of a lubricant absorbent ceramic as an effective self-supporting reservoir, to overcome the need for a rigid enclosure. The outer surface of the ceramic material is coated with a glaze compound to retain the lubricant and to prevent its escape. The surface of the ceramic material that is in contact with the bearing is unglazed so that lubricant is fed by capillary action.
When pivoted mechanical elements became widely used, there were other more pressing problems that required solution first, as suggested by prior U.S. Pat. No. 3,227,250 in its teaching of a readily disconnectable pivot connection. Even its suggestion of the use of a Teflon varnish for the purpose of lubricating the pivoted element has the primary reason for such use to facilitate making the connection readily disconnectable.
A somewhat later effort to solve the problem of providing a reservoir to dispense a lubricant to a moveable bearing is revealed by a French Pat. No. 2,345,618 published Oct. 21, 1977. That prior patent teaches using a woven fabric of synthetic material impregnated with easily ruptured micro capsules in its pores to break progressively during use thereby releasing lubricant to the bearing.
More recently, U.S. Pat. No. 4,290,655 provides an improvement in sintered self lubricating bearings by forming a porous sleeve with areas having different permeability for a lubricating oil to control the free circulation of the oil within the sleeve bearing of a continuously rotating shaft.
More closely associated with the environment in which the present invention is cast is U.S. Pat. No. 4,308,794. This prior patent discloses a print hammer with an internal cavity containing felt that is impregnated with lubricating oil. The cavity is sealed except for two holes through which the oil seeps to lubricate the print hammer.
While disclosing a striker mechanism for a high speed printer, a prior U.S. Pat. No. 4,430,936 suggests using oil impregnated felt washers arranged coaxially to provide lubrication for the pivot pin of the moving element. Primarily, however, the disclosure of this prior patent describes the use of a felt pad impregnated with oil to function as a damper element for eliminating the vibrations of a print hammer and for reducing the settling time of the striker mechanism after the printing of a character.
The following prior publications are IBM Technical Disclosure Bulletins: Vol. 19, No. 8, Page 3109, dated January, 1977; Vol. 24, No. 1A, Pages 20 and 21, dated June, 1981; Vol. 24, No. 3, Pages 1420 and 1421, dated August, 1981; and Vol. 28, No. 12, Pages 5247 and 5248, dated May, 1986.
None of these IBM Technical Disclosure Bulletins and none of the above-identified prior art patents disclose or even suggest a solution to the problem of maintaining the lubrication of a high speed print hammer pivot pin over an extended period of time for increasing its useful life even further, as that provided by the present invention.
Present day high speed printers have achieved such high speeds of operation that wear of the hammer pivot pin has become a major factor in determining the useful operating life of such printers. Repairs are difficult as is a re-lubrication program for the print hammer pivot pins due to several reasons including the small, compact size of these elements.
It has been learned that these print hammer elements, due to their high speeds of operation, develop positive and negative pressures alternately on the pivot pins, which cause them to act as small pumps, actually sucking the lubricant out of the bearing reservoir. This action results in premature failure of print hammer pivot pins at today's high cyclical speeds of operation.
The pivot pins of print hammer elements customarily are made of sintered material that is impregnated with lubricant, and actually, such pivot pins provide very economical means of lubrication without requiring additional parts and extra space in the tightly-packaged areas in low speed print hammer modules. The only problem realized with this means of lubrication at todays higher speeds is the limited life of the pivot pin, typically in the order of 150 million cycles of operation.